Cell and Cell Division
Topic
Page No.
Exercise
Notes
02 - 08
Cell Division
Amitosis
Cell Cycle
Mitosis
Meiosis
Difference between meiosis & mitosis
09 - 09
09 - 09
10 - 11
11 - 13
13 - 18
18 - 19
Syllabus
Cell and Cell Division
Cell Division, Amitosis, Cell Cycle, Mitosis, Meiosis, Difference between meiosis
& mitosis
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Cell and Cell Division
LEVEL - I
1.
During mitotic anaphase, choromatids migrate(A) From the poles of the cell towards the metaphasic plate
(B) Toward the NM
(C) Along with their side chromatids towards the one pole
(D) From metaphase towards the poles
2.
Microtubeles thats form the spingle tend to originate from ________ and terminate in _________.
(A) Centromeres, telomeres
(B) Centrioles, telomeres
(C) Centrioles, kinetochores
(D) The nuclear envelope, kinetochore
3.
Synaptonemal complex is more conspicious at
(A) Zygotene
(B) Pachytene
4.
(C) Diplotene
In plant cell, cytokineses is accomplished by the formation of a(n)(A) Aster
(B) Equatorial plate
(C) Cell plate
(D) Leptotene
(D) Membrane furrow
5.
The distribution of mitochondria and plastid between the daughter cells during cytokinesis(A) Is random
(B) Is directed by centrioles
(C) Is directed by mitotic spindle
(D) Is directed by nuclear membrance
6.
When dividing cells are examined under a light microscope, chromosomes first become visible during(A) Interphase
(B) Prophase
(C) Metaphase
(D) G2 and S phases
7.
Centrosomes are (A) Constricted regions of chromosomes(B) Regions where microtubules polymerise
(C) Part of Cilia
(D) The central part of the same cell
8.
In the cell cycle, mitosis occurs between (A) G1 and S phase
(B) S and G1 phase
(C) S and G2 phase
(D) G1 and G2 phase
9.
Celluler structures which disappear during late prophase and reappear during anaphase are (A) Mitrochondria and spingle fibers
(B) Plasma membrane
(C) Nuclear membrane and nucleolus (D) Plastid
10.
The best stage to observe shape, size and number of chromosomes is (A) Metaphase
(B) Anaphase
(C) Interphase
(D) Prophase or telophase
11.
A picture of dividing Drosophila cell through a microscope show that the cell contain 8 chromosomes, each consisting of two chromatids. The picture must have been taken during(A) Mitotic metaphase
(B) Mitotic Anaphase
(C) Meitotic Anaphase II
(D) Meiotic Telophase II
12.
Which of the following phase of mitosis is essentially the reverse of prophase in term of nuclear
change ?
(A) S-phase
(B) Anaphase
(C) Telophase
(D) Interphase
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13.
Which of the following is not a function of mitotic cell division in animals ?
(A) Growth and repair
(B) Cell replacement
(C) Asexual reproduction
(D) Gametes formation
14.
Given 1. Chromatic
2. Monod
3. Dyad
4. Daughter chromosome
The correct sequence in cell division is :
(A) 1, 2, 3, 4
(B) 2, 3, 1, 4
(C) 3, 2, 1, 4
(D) 4, 3, 2, 1
15.
Cytokinesis in animal cell takes place by _________ ; in __________ direction while in plant cell
by ____; in ______ direction (A) Furrowing, centrifugal, cell plate, centripetal (B) Furrowing, centripetal, cell plate, centrifugal
(C) Cell plate, centrifugal, furrowing, cetripetal (D) Cell plate, cetripetal, furrowing, centrifugal
16.
Phragmoplasts or Phragmosomes, vesicles of GB are the precursors of (A) Cell plate
(B) Chloropalsts
(C) Chromoplast
(D) Protoplast
17.
In some organisms like certain fungi and algae, cell undergoging mitosis repeatedly without sbsequently
undergoging cytokinesis. What would result from it ?
(A) Decrease in the chromosomes number
(B) Decrease int eh DNA molecules
(C) Formation of syncytium
(D) Multicellularity
18.
Liqiud endosperm in coconut is resulted due to (A) Karyokinesis followed by cytokinesis
(B) Failure of karyokinsis followed by cytokinesis
(C) Karyokinesis twice followed by single cytokinesis
(D) Karyokinesis is not followed by cytokinesis
19.
Which of the following statements is correct ?
I. Mitosis / equitorial is usually restricted to diploid cells
II. In some lower plants and in some social insects haploid cells also divided by mitosis
III. Mitosis is essential for the cell to restore the nucleos-cytoplasmic ratio
IV. Mitotic division in apical meristem lateral cambium results in a continuous growth throughout the
life
(A) All are correct
(B) All are incorrect
(C) Only I and are correct
(D) Only IV are correct
20.
Significance of mitosis is in (A) Producing idnetical cells genetically similar to mother cell
(B) Increasing cellular mass
(C) In reducing the chromosome (2N  N)
(D) Swift division
Answer Key (Level - I)
1.
7.
13.
19.
D
A
D
A
2.
8.
14.
20.
C
B
C
A
3.
9.
15.
B
C
B
4.
10.
16.
C
A
A
5.
11.
17.
A
A
C
6.
12.
18.
B
C
D
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LEVEL - II
1.
To form 16 cells how many times mitosis is supposed to occurs (A) 4
(B) 15
(C) 16
(D) 2
2.
The members of a homologous pair of chromosomes (A) Are identical in size and appearance
(B) Contain identical genetic information
(C) Separate to opposite poles of the cell during mitosis
(D) Are found only in haploid cells
3.
The four haploid nuclei found at the end of motisis differ from one another in their exact genetic
composition, some of this difference is the result of
(A) Cytokinesis
(B) Replication of DNA during the S-phase
(C) Spindle formation
(D) Crossing over during prophase I
4.
A cell that passes the restriction point ? will most likely (A) Undergo chromosome duplication
(B) Have just completed cytokinesis
(C) Continue to divide only if it is a cancer
(D) Move into the G0 phase
5.
Asexual reproduction produce genetically identical individuals because (A) Chromosome do not have to replicate
(B) It involves chromosomes replication without cytokinesis
(C) No meiosis or fertilization takes place
(D) The only cell division that occurs in meiosis
6.
One difference between mitosis and meiosis I is that (A) Chromosomes do not replicate in the interphase preceding meiosis
(B) Chromosomes do not replicate in the interphase preceding meiosis
(C) Homologous chromosomes pairs synapse during meiosis but not mitosis
(D) Spindle composed of microtubules are not required during meiosis
7.
The number of chromosomes is reduced to half during (A) Anaphase of mitosis and meiosis
(B) Meiosis II
(C) Meiosis I
(D) Fertilization
8.
Which of the following is correct ?
(A) DNA replication occur before mitosis but not meiosis.
(B) In human cells, chromosome are visible only during mitosis and meiosis.
(C) During meiosis centromere divides in anaphase I.
(D) Crossing over takes place in meiosis II.
9.
Chromosome number is reduced during meiosis because the process consisting of (A) Two cell divisions without any chromosome replication
(B) A single cell division without any chromosome replication
(C) Two cell divisions in which half of the chromosomes are destroyed
(D) Two cell divisions and only a single round of chromosome replication
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10.
Which of the following is false ?
(A) The mitotic spindle is composed of microtubules
(B) In plant cells, cytokinesis is accomplished by the formation of a cell plates
(C) The distribution of mitochondria between the daughter cells during cytokinesis is random
(D) Chromatin condenses to form discrete, visible chromosomes early in G1
11.
A reduction step during meiosis is important because (A) It returns the chromosome number to normal before fertilization
(B) There is a mechanism for this
(C) Only one copy of each chromosome is necessary
(D) Otherwise chromosomes copies would double each fertilization
12.
The important of synapsis and the formation of chiasmata is that (A) Reciprocal exchange of chromosomal sections occurs
(B) The DNA on homologous chromosomes mix
(C) As a result an increase in the variation of progeny occurs
(D) All of the above
13.
Which of the following is true for the meiosis?
(A) The chromosome number in the resulting cell is halved
(B) DNA replication occurs between meiosis I and meiosis II
(C) The homologous do not pair during prophase I
(D) The chromosome number of the resulting cells in the same as the that of the parent cell
14.
Which of the following statements about meiosis is false ?
(A) Haploid cells cannot divide meiotically
(B) DNA replication does not occurs during interkinesis
(C) Assortment of homologous chromosome on the equatorial plate during metaphase I is independent
(D) Of the four cells produced by meiosis, none are genetically identical to the parent cells but two
are indentical to each other.
15.
Which one is correct about meiosis ?
(A) First division is reduction and second division is equational
(B) The best material for the study of meiosis is micropore (pollen) mother cell in microsporangium
of anther
(C) The important of meiosis lies in maintaing the chromosomal number from generation to generation in sexually reprodducing organisms
(D) All
16.
Which one is correct about crossing over/genetic recombination ?
I. It occurs in tetrad stage. It occurs in pachytene of prophase I of meiosis I
II. It occurs between non-sister chromatids of homologous chromosomes
III. It is the exchange of genetic material between the homologous chromosomes
IV. It is recombinase enzyme mediated process
V. Recombination nodule of synaptonemal complex between the homologous chromosomes is the
sire of crossing over.
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VI. It causes genetic variation which is important for evolution.
VII. It is also the reciprocal transfer of genes between the non homologous chromosomes
(A) All
(B) All except VII
(C) II, IV and VII
(D) I, III and VII
17.
18.
Recombination is caused by (A) Crossing over alone
(C) Independent assortment only
(B) Crossing over and independent assortment
(D) Chiasma formation
I. It is the final phase of prophase I
II. Termination of chiasmata occurs
III. Fully condensed chromosomes
IV. Spindle is assembled
V. Nucleolus and NM breaks down
The above characteristic are attributed to (A) Zygotene
(B) Diakinesis
(C) Diplotene
(D) Metaphase I
19.
Anaphase I is marked by (A) Movement of homologous chromosomes toward their respective pole
(B) Centromere division
(C) Sister chromatids remain associated at centromere
(D) A and C
20.
Which one is correct about bivalent ?
I. Bivalent are tetrads
II. A bivalent means 4 chromatids and 2 centromere
III. One bivalent consists of 2 homologous chromosomes, each and sister chromatids
IV. Bivalents form in zygotene
(A) All
(B) Only III
(C) III and IV
(D) Only IV
Answer Key (Level - II)
1.
7.
13.
19.
A
C
A
D
2.
8.
14.
20.
A
B
D
A
3.
9.
15.
D
D
D
4.
10.
16.
A
D
B
5.
11.
17.
C
D
B
6.
12.
18.
C
D
B
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LEVEL - III
1.
Four different steps that occur during meiosis are given in the following list
I. Complete separation of chromatids.
II. Pairing of homologous chromosomes.
III. Lining up of paired chromosomes on equator.
IV. Crossing over between chromatids.
These steps would occur in the order (A) II, III, IV, I
(B) III, II, IV, I
(C) II, IV, III, I
(D) III, I, II, IV
2.
The DNA content of individual cells and the number of cells in each phase of a “cell cycle” can be
determined using flow cytometry. Which of the following combination of “phase of a cell cycle and
its corresponding DNA content” can be considered normal?
A. Diploid cells found in the ? or ? phase
B. Cells with twice the normal DNA content in the early M phase.
C. Cells with intermediate amounts of DNA content in the ? phase.
D. Cells with twice the normal DNA content in the ? phase.
(A) A and B
(B) B and C
(C) C and D
(D) All
3.
Sara would like to film the movement of chromosomes during cell division. Her best choice for a
microscope would be
(A) light microscope, because of its resolving power.
(B) transmission electron microscope, because of its magnifying power.
(C) scanning electron microscope, because the specimen is alive.
(D) light microscope, because the specimen is alive.
4.
Let a cell`s generation time is 1 minute. In 20 minutes a culture tube (culture medium) is 1/8th filled
with cells. When the tube will be fulfilled?
(A) 21 minutes
(B) 23 minutes
(C) 60 minutes
(D) 160 minutes
5.
Number of chromatids at metaphase is :
(A) Two each in mitosis and meiosis
(B) two in mitosis and four in meiosis
(B) Two in mitosis and one in meiosis
(D) one in mitosis and two in meiosis
6.
How many equation divisions are necessary in a cell of onion root tip to form 128 cells ?
(A) 64
(B) 128
(C) 7
(D) None of these
7.
You are given 13 meiotic divisions. From these division how many seeds you can form ? How many
pollen grain will be wasted ?
(A) No. of seed - 13, No. of pollen grain - 13
(B) No. of seed - 10, No. of wasted pollen grain -2
(C) No, of seed - 2, No. of pollen grain - 10
(D) No. of seed - 52, No. of pollen grain - 52
8.
What would be the number of chromosomes of the aleurone cells of a plant with 42 chromosomes in
its roots tip cells ?
(A) 21
(B) 42
(C) 63
(D) 84
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9.
Select the correct option with respect to mitosis
(A) Chromosomes move to the spindle equator and get alingned along equatorial plate in metaphase
(B) Chromatids separate but remain in the centre of the cell in anaphase
(C) Chromatids start moving towards opposite poles in telophase
(D) Golgi complex and endoplasmic reticulum are still visible at the end of prophase
10.
At metaphase, chromosomes are attached to the spindle fibres by their :
(A) Satellites
(B) Secondary constrictions
(C) Kinetochores
(D) Centromere
11.
Meiosis results in
(A) Production of gametes
(C) Introduction of variation
(B) Reduction in the number if chromosomes
(D) All of the above
12.
Meiosis occurs in organism during
(A) Sexual reproduction
(B) Vegetative reproduction
(C) Both sexual and vegetative reproduction
(D) None of the above
13.
During anaphase - I of meiosis
(A) Homologous chromosomes separate
(B) Non-homologous autosomes separate
(C) Sister chromatids separate
(D) Non-sister chromatids separate
14.
Meiosis is characterized by
(A) Double reduction division
(C) Both reduction and equal division
15.
(B) Equal division only
(D) Homotypic division
Cells which are not dividing are likely to be at
(A) G1
(B) G2
(C) G0
(D) S phase
Answer Key (Level - III)
1.
7.
13.
C
D
A
2.
8.
14.
D
B
A
3.
9.
15.
D
C
C
4.
10.
B
A
5.
11.
A
A
6.
12.
C
A
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CELL DIVISION
Every cell has the fundamental capability to divide. This led Virchow to make the famous conceptual
statement Ominis cellule (exception viruses), i.e. cells originate from pre-existing cells. Cell division
and reproduction has evolved over 4 billion years into a very precise, flaw less and efficient mechanism. In fact it is a very intricate, sophisticated and controlled process involving biosynthesis, assortment and distribution of thousand of biomolecules. Cell proliferation facilitates replacement, growth,
repair , regeneration and maintenance of cell population. A healthy human male weighing 70 kg has
1614 RBC which are derived from the single called zygote which during development undergoes
extensive cleavage. Likewise, o homeostatically maintain a population of RBC (4.5-5.0 million/cm3)
in blood, the haemopoietic tissue (bone marrow) have to maintain a turnover rate of 2.5 × 106 cells/
second. Similar activity occurs to replace epithelial cells (e.g. skin) which diet the rate of several
millions/day. Reproductive cells particularly sperm are produced in millions. They are ejaculated in
one intromission in several millions.
Cell division is of 3 types.
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AMITOSIS
In primitive, single called organisms such as bacteria, protozoa, and in the cells of the foetal membranes of many amniotes, division of cells occurs by constriction of cytoplasm followed by spliting of
nucleus. The main stages are as follow :
(a) Nucleus elongates and assumes a dumb-bell shape.
(b) A constriction appears in the middle of elongated nucleus dividing it into two nuclei
(c) The cytoplasm also exhibit construction and this process is completed after formation of daughter
nuclei.
(d) No chromosomal division occurs and this facilitates formation of daughter cells of similar size.
(e) This division is characterized by no significant event of change in the nucleus.
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Cell Cycle
The interphase stage which exists between two mitotic division is not a ‘quiescent state’ as was
believed earlier. Daniel Mazia and others showed that although in this phase there is no division of
chromosomes and cytoplasm yet considerable anabolic. catabolic activities occur which leads to
augmentation of cell volume. Further, interphase is of longest duration as well.
Cell Cycle consists of four well defined phases :
G0 No significant metabolic activity occurs during this Quiescent period. In fact it is the initial stage
of G1 (held at special point where the process of cell division is inhibited.
G - 1 Phase is marked by storage/synthesis of a variety of enzyme, precursor nitrogenous bases,
pentose sugar, phosphoric acid, ATP, Mg2+ /Ca2+ needed, for DNA replication. The duration of this
stage depends on the physiological status of cell. This stage occurs between the end of one nitotic
division and DNA biosynthesis of another. In some cells its duration is 10 hours (4-9 hours, duration
40-50 %)
S - Phase is characterized by duplication of chromosomes. Thus each chromosome has a similar
copy. Histone synthesis also occur and they become associated with replicated DNA (new).
In some cells duration of this stage is about 9 hours (5-7 hours; 30-45 %)
G - 2 Phase It is also called post- DNA replication stage. It is marked by increase in volume of
cytoplasm, macromolecules, and cell organelles (duration in % is 10-20 %).
M - Phase This is the stage of initiation of cell division and comprises of (a) Prophase, (b) Metaphase,
(c) Anaphase, (d) Telophase. Significant changes occur in nuclear membrane, nucleolus, chromosomes, centriole and other organelles. A variety of enzymes; protein; ATP, Ca2+/Mg2+ etc. are involved in these intricate mechanism (Duration 5-10 %)
Interphase
1. Nucleus enveloped completely by nuclear membrane
2. Chromosomes long and coiled, chromatids threads are not distinct
3. Amount of DNA is doubled (in- S-phase)
4. Nuclear volume increase due to accumulation of rRNA and ribosomal proteins.
Duration of cell cycle in some typical cell
S. No
Cell Type
1 Hela cell line
2 Leucocytes
3 Meristem cells of root of Vicia faba
G1
14
21
6
S
8
28
6
Cell Cycle
G2
5
6
5
M
1
2
1
Total
28
57
18
* Same in kidney cells of Frog.
Mitotic cell division is regulated by a variety of specific proteins. Phosphokinin and cyclin act
initiator of mitosis. A matutation promoting factor (= specific protein) is biosynthesized at the end
G2-phase of cell cycle and before prophase. In plants, gibberellins and cytokinins act as mitogenic
substances.
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MITOSIS
1.
2.
It occurs in somatic as well as in germ cells. It comprises of two stages.
A very intricate nuclear division called karyokinesis.
Division of cytoplasm called cytokinsis.
Mitotic cell division ensures distribution of exact copy of DNA, cell organelles and large biomolecules
to daughter cells. No change occurs in the shape, size number of chromosomes and in the sequence
of genes. Thus a daughter cell can be called an exact copy of parental cell in as far as chromosome
number is concerned.
During mitosis, significant and complex changes occur in
(a) nuclear membrane,
(b) chromosomes
(c) nucleolus
(d) cell organelles.
There are four main phase of mitosis. They are
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Prophase
1.
2.
3.
4.
5.
6.
7.
The distinguishing features of this stage are :
Cells becomes spherical and cytoplasm becomes refractive and viscous. Nuclear membrane begin
Nucleous breaks and the broken fragments are dissoluted.
Chromosomes progressively become large and restricted.
Mitotic spindle is formed by microtubules present in cytoplasm. (not in plant cells). In animal cells,
the centriole divides into two (diplosome). Each centriole then migrates to opposite poles and
replicate.
Chromosomes are condensed. Each chromosome comprises of two chromatids which are bound
together by centromere. They called sister chromatids.
Chromatids are attenuated in size and are thick.
The centriole present between 2 centriole. very fine nicrotubules (= aster) extend from it and form
the spindle.
Metaphase
1.
2.
3.
4.
5.
formation of a mitotic spindle which consist of 3 types of fibres (i) chromosomal (ii) continues (iii)
inter zonal.
All the chromosomes migrate to equatorial region.
Chromosomes appear very distinct can be easily counted.
Nuclear membrane disappears completely.
Nucleolus disintegerates and disappears.
Anaphase
1.
2.
3.
4.
5.
The centromere divides to initiate the first significant change of this stage, (due to DNA replication
in this region).
The spindle fibres pull the sister chromatids to opposite poles. This force of repulsion is so strong that
it snaps the connection of centromere with the bound sister chromatids.
According to the position of centromere chromosomes acquire special forms, e.g. J-shape
(submetacentric), rod-shaped (telocentric) or V shaped (metacentric)
The chromotids and two chromosomes are formed.
Chromosomes becomes short and thick; and migrate to opposite poles of cells due to contraction of
chromosomal fibres and inter zonal fibres.
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Telophase
1.
2.
3.
4.
5.
This is the last stage of mitosis and can be easily identified b the following features:
Collection of homologous chromosomes at opposite poles.
Chromosomes becomes decondensed, and uncoil to acquire a form similar to one seen in interphase.
Degeneration of mitotic spindle.
Biosynthesis of nucleolus.
Formation of nuclear membrane around chromosome resulting in the formation of two daughter
nuclei.
Cytokinesis
In animal cells, division of cytoplasm occurs by cleavage furrow. This process is initiated concomitantly with telophase and is accomplished along with the formation of daughter nuclie. This results in
the formation two of daughter cells. DNA replication and cytokinesis are two separate and independent events.
In plant cells, the cell plate made up of pectin is formed between cells and facilitates division.
Subsequently, cellulose and other chemicals (e.g. lignin, mucopolysaccharide etc.) join with cell wall.
The two daughter cells are not of the same size. Further, they do not receive equally the cytoplasm
and cell organnels. However, they have similar genetic constitution since they similar (a) number of
chromosomes and (b) genetic capabilities.
Significance of Mitosis
1.
2.
3.
All the cell of the body have the same number of chromosomes. Mitosis ensure that this trait does
not alter.
Mitosis ensures growth of tissues/organs by increasing the number of cells.
There is no change in the genetic capability due to mitosis.
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MEIOSIS
All biological organism (except viruses) have fixed number of chromosomes. Thus, the diploid
number (2n) of chromosomes in human in 46 (= 23 pairs); in Drosophila melanogaster 8; in pisum
sativum
Sexually reproducing animal produce sperm (by spermatogenesis) and ovum (by oogenesis) which
unite to form zygote. The sex cells (sperm and ovum) contain half the chromosomes present in
somatic cells, e.g. human sperm and ovum has 23 (n) chromosomes each. When these sex cells
unite the ‘2n (diploid) number i.e., 23 + 23 = 46 ( or 23 pairs) is restored. This means that during
spermatogenesis and oogenesis reduction division occur which halves the chromosomes number
(2n  n). Thus, reduction division is just reverse of fertilization (n + n = 2n).
Meiosis or reduction occurs only in seminiferous epithelium of testes in male, and in the germ cells of
ovary in female. In plant, these occur in pollen mother cells (present in microsporangia of anther
lobe) and in megasporangia of carpels. The end product of these are pollen grains (= sex cell of
male); and ovule (sex cell of female).
During meiosis chromosome divide only once but cytokinesis occurs twice. Thus, there are two
divisions during meiosis.
1.
2.
3.
4.
Meiosis I (Reduction division) also called heterotypic
Meiosis II (Equational division) = homotypic
Meiosis occurs in animal and plant cells. It is categorized into 3 types
Sporogenetic meiosis occurs during spore formation in only some plants.
Gametic meiosis occurs in majority of animals and lower plants during gametogenesis.
Zygotic meiosis occurs in lower plants in egg soon after fertilization.
Animal cells that undergo meiosis are called meiocytes. In male, these are primary spermatocytes
of testes and in female primary oocytes.
Meiosis is triggered by a variety of factors in animals.
Balance of nuclie acids in nucleus
Some parasitic flageppate protozoans undergo meiosis in response to stimulating action of insect
hormone called ecdysone.
Imbalance in amounts of DNA (more) and RNA (less).
Meiosis initiating factors.
1.
Meiosis I ( = Reduction division) = Heterotypic
2.
3.
4.
It has 4 main phases:
(a) Prophase I
(c) Anaphase I
Meiosis I 1. Prophase I
(i) Pre-Leptotene
(iii) Zygotene
(v) Diplotene
Metaphase - I
Anaphase - I
Telophase - I
I.
II.
III.
1.
2.
(b) Metaphase I
(d) Telophase I
(ii) Leptotene
(iv) Pachytene
(vi) Diakinesis
Cytokinesis- leads to the formation of 2 haploid and genetically different daughter cells. Before the
start of Meiosis I (M-I) the cells are in the interphase stage where DNA replication occurs (in S
stage only) by semi-conservative process. Two double-stranded DNA are produced from one double
stranded DNA. In this there is one earlier strand and one new synthesized strand. Each
chromosome has several replicating units called replicon. DNA synthesis occurs simultaneously in
both strand in 5' 3' direction. Small fragments of DNA are synthesized and are joined by Ligase.
The synthesis of DNA coincides with synthesis of histones.
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Prophase I
1.
2.
3.
It is the longest stage. It is divided into the following six sub stages.
Pre-Leptotene
Can be identified by the following features
Chromosome have two strands. This is due to DNA replication in interphase.
Nuclear volume is large
Chromosomes are filamentous. Only sex chromosomes are visible. ( not autosomes)
Leptotene
1.
2.
3.
4.
Chromosomes are easily distinguishable and their diploid state is clear in many animals.
Chromosomes contain serially arranged granular structures called chromomeres.
Each chromosome contains an axial filament around which chromatin fibres are attached in
sequence of lateral loops.
Homologus chromosomes are attached to nuclear membrane.
Zygotene
1.
2.
3.
Chromosomes become shorter and thicker.
Homologus chromosome synapse length-wise. One of the homologus (of the pair) is of paternal
origin and the other maternal origin.
The homologus chromosomes are bounde all along their length. When the pairing starts from the
centromere and extends upto the other end it is called Procentric.
Pairing may occur in homologus regions of chromosomes. When this occurs synaptonemal
complex can easily seen.
Pachytene
1.
2.
3.
4.
5.
6.
This is longest stage f prophase-I in the spermatogenesis of mammals.
The chromosome appear as bivalents. The number of bivalents is half of diploid number of
chromosomes.
The chromosomes are so condensed that appear in the form of thick threads. This change of form
is further faciliated by longitudinal contraction.
The two homologus chromosomes of a bivalent become coiled together. Homologus chromosome
is made up of two chromatids which are connected with each other by centromere.
Crossing over of genetic material occurs between two homologus chromosomes.
Genetic exchange occurs via very specific sites of synaptic chromosome called chiasmata.
Diplotene
1.
2.
3.
4.
Synaptic chromosomes separate.
Terminalisation occurs during the stage. The terminal chiasmata is displaced and its place is taken
up by interstitial chiasmata become smaller. At the end of terminalisation process the homologus
chromosome remain in contact with each other via terminal chiasmata.
The synaptonemal complex degenerates and disappears.
A chromatoid body make its appearance.
Diakinesis
1.
2.
3.
4.
Chromosomes becomes more constricted.
The bivalent chromosome migrate to periphery of the nucleus.
The homologus chromosome maintain their contact via terminal chiasmata.
Nucleolus dissociates from chromosome and disappear.
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Metaphase -I
1.
2.
3.
4.
5.
6.
Nuclear membrane disappears
The chromosomes are excessively constricted
The process of spindle formation is initiated and completed
The chromosomes orient themselves in the equatorial region of the spindle.
The spindle fibres attach to centromere of two homologus chromosome in opposite direction.
Active repulsion between homologus chromosome lead to their separation.
Anaphase - I
1.
2.
3.
Each homologus chromosome migrates to opposite poles of the cell. In this stage each chromosome
is made of two chromatids. Migration of chromosomes is irregular.
Chromosomes do not separate together. Small chromosome separate rapidly while longer ones take
more time to do so because of the presence of internal chiasmata
Extensive genetic changes occur in the genetic make up of chromosomes due to crossing over.
Telophase - I
1.
2.
3.
The chromosome migrate to opposite pole and remain in a condensed state for sometime. After
some time they begin to undergo despirilisation and become long.
Nuclear membrane is reconstituted.
Nucleous is not synthesized.
In animal cell, karyokinesis in telophase ends with initiation of cytokinesis and its completion. The
resulting daughter cells (2) have haploid number of chromosomes (n).
Meiosis II
This is almost similar to mitosis. In this centromere duplicates but not DNA. There is a short
interphase but no duplication of DNA occurs in it.
Prophase II
1.
2.
3.
4.
It lacks the complexity of chromosome and nuclear behavior as seen in Prophase- I
The arms of chromosome are longer in this stage.
Spindle is formed.
Nuclear membrane disappears.
Metaphase II
Chromosomes are arranged on equatorial plate. All other features are similar to Metaphase of
mitosis.
Anaphase II
1.
2.
Division of centromere take place.
The two sister chromatids of a chromosome migrate to opposite poles of cell. After their separation
they assume the form of a chromosomes.
Telophase II
1.
2.
3.
Reconstruction of nucleus in a manner similar to mitosis.
Nucleus, centrole, and chromosomes enter interphase stage.
Each nucleus contains haploid number of chromosomes.
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Significance of Meiosis
1.
2.
It maintains the constancy of chromosome number in sexually reproducing organisms.
Genetic variation in organisms due to exchange of genetic material (crossing over) during synapsis
of homologus chromosomes. Such variation is the base of evolutionary developmental process and
facilitate emergence of new strains/variates/species consequent to Natural/artificial selection.
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DIFFERENCE BETWEEN MEIOSIS AND MITOSIS
Meiosis
Mitosis
1.
Cells divides twice
1.
Divides only once
2.
It occurs in organsims
that produce sexually
2.
Occurs in all cells of body
of organisms irrespective of
of whether they reproduce
sexually or asexually
3.
DNA replication occurs only
in Interphase-I and never in
interphase-II
Meiosis
3.
DNA replicate once in inter
phase
4.
Prophase is long (of several
days), complex and has many
substages. Pre-lepotene,
lepotene, zygotene, pachytene,
diplotene and drakinesis.
4.
Prophase is only of few
hours duration. It has no
substages
5.
Cell divides twice but chromosome divided only once
5.
Cell and chromosomes
divide only once.
6.
Pairing (Synapsis) of homologus
chromosomes occurs during
Zygotene.
6.
No synapsis.
7.
Synaptic pair during pachytene
exchange genetic material (glued
chromosome segment) via
chiasmata.
7.
No such exchange occurs
between chromatids
8.
The 2 homologus bivalent form
tetrad (4 chromatids and 2
centromere).
8.
Each chromosome has 2
sister chromatids attached
by centromere.
9.
Centromere divide only in Anaphase II and not in Anaphase I
9.
Centromere divided in
anaphase
10.
Smaller chromosome separate
before longer ones.
10.
Chromosome separation
occurs simultaneously
11.
Spindle fibres do not disappear
completely in Telophase- II.
11.
Disappear completely in
telophase
12.
Nucleolus is not formed after
Telophase-I.
12.
Nucleolus is resynthesized
Telophase
13.
2n 
13.
2n 
14.
The genetic constitution of
daughter cell is different than
parental cell due to crossing over.
14.
Daughter cells have the
same genetic constitution as
parent cell.
n
Mitosis
n
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